OP_X

The OP_X op codes add a numerical value to the top of the stack.
OP_0 pushes an empty byte array which is interpreted as the numerical value 0.
The rest push a single byte to the top of the stack as a two’s complement integer.
OP_1 pushes a 1, OP_2 pushes a 2, and so on through OP_16, which adds a 16 (0x10).

OP_TRUE and OP_FALSE

OP_FALSE is an alias for OP_0 while OP_TRUE is an alias for OP_1.