What is the meaning of concatenate

Answer 1

Concatenation is "joining"* or "glueing" several parts of a query together:
SELECT CONCAT("Sololearn", "is", "fun!") AS Solo;

* - not to be confused with JOIN ;)

Answer 2

concatenation means joining or merging together of two or more things.
you can concatenate arrays I.e joining two or more arrays into one
you can concatenate strings e.t.c

Answer 3

it means combining statements