Knowing some history is always useful for understanding peculiarities such as the charge of the electron.
Understanding the electricity developed gradually during a long time. In the 18th century it was only a wild assumption that there's some special ingredient (=the charge) in materials which causes phenomenons we call electricity. But that assumption glued together several phenomenons and gave a possibility to make plausible speculations of what there actually happens.
Experiments and the assumption of the existence of charge together forced physicists to decide that there's 2 types of charge; the types are plus and minus. In addition new charge cannot be created, materials contain it some fixed total amount. Electrically neutral materials contain as much plus and minus charges. No idea, how much, but as much both of them. And it's possible with different methods to move charge from one piece to another and see, how the pieces push or pull each other depending do they have the same or opposite net charge polarities.
It was noticed soon that the charge can flow somehow easily in metals and some liquids, but there are also insulating materials - solids, gases and liquids - where the charge doesn't move except in cases there's enough charge difference at so short distance that the insulating material breaks down.
The force between different polarity charges also gave a possibility to measure the amount of charge. Experiments made clear that that the amount charges can be compared by assuming the repulsion force between two charged pieces obeys Coulomb's law ( a Wikipedia image):
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k is constant. In modern SI unit system k =8.988×109 N⋅m2⋅C−2, but that number was not known in 1700's.
Another as wild assumption as the existence of charge was to assume, that chemical phenomenons are somehow bound to the behaviour of charge. Today (=200 years later) we know that all chemical phenomenons happen just because electrons jump from their orbits of an atom to other orbits caused by another atom.
Physicist Ampere investigated how one could measure how much charge flows through a conductive piece in a given time period. His experiments finally showed that a good measure for the current is how much metallic silver is separated per second from a liquid solution which contains silver as its only metal. Ampere assumed that charge moves as bound to the metal which flows in the liquid due the electric force.
Ampere essentially coated something with silver by using the already known electrochemical metal coating method (=electroplating), but his actual invention was that it could be used to measure the current. No other exactly repeatable method based on as few assumptions was known then.
Atoms and electrons were also wild assumptions until physicists showed with experiments that charge occurs in certain minimum portions either +e or -e and those minimum portions are contained in certain atomic particles that we call protons and electrons.
In SI unit system e= −1.602176634×10^-19 ampereseconds.
An unfortunate accident was that the direction of electric current was defined with Ampere's principle (=charge travels with moving silver) and much later it was discovered that current in metals is mostly moving electrons. For this reason most of us have struggled a while when we were first time told that current goes in a wire to the right when the electrons go to the left in the same time. But that couldn't be changed because electric industry was already a multi billion dollar business when the existence of electrons was finally accepted.
But what is that so called charge that we claim to be contained amount -e in an electron?
It cannot have any mechanical explanation, because all mechanical we can see is very much based on the behaviour and properties of electrons. We must accept that the charge is one of the main properties of electrons, something which is opposite than what protons have and what does not occur in neutrons. Another quite as well (or as little) known property of electrons is the mass and it's as well impossible to be explained somehow better mechanically. It's mass and that's it. Only atomic particle physicists have finer theories of what ingredients an electron contains.