A pseudo-instruction doesn't actually exist in the instruction set of a processor. A pseudo-instruction will be a convenient single name for one or more actual instructions.
A common example is the unconditional jump instruction.
Normally the syntax for this instruction would be:
...but the assembler might actually translate that into:
cmp t0 r0 r0
jmp t0 address
Which is basically checking to see if the zero register is equal to the zero register, and if so jump. Since this will always be true, it will always jump.