A positive electric charge is an electric charge which is of the same sign as the electric charge on a proton.
When it is necessary to assign a value to a positive electric charge, a $+$ (plus) sign is used, and the value assigned is a positive number.
It appears at first glance that the definition of the sign of an electric charge is both arbitrary and circular.
Early in the development of the theory of electrostatics, it was noticed that there were two types of electric charge.
What they were called was at that point arbitrary; one type was called positive, and the other type was called negative.
When the proton and electron were discovered, the signs of their electric charges were identified, and subsequently used to define the sign of an arbitrary electric charge.
Hence the fact that a proton is positive and an electron is negative is simply the result of a convention.