I'm gonna explain it to you. It's just slangy for "going to".

gonna...means go to

It really looks terrible in written English! Use it only if you're quoting someone, otherwise you look uneducated. ;) Spoken English cuts corners, especially the more casual the situation is. There's great examples above (thanks fdmaxey!). Other variations on 'gonna' are: "I'ma" ('I am going to', eg. I'ma do that after) and " 'mona " (again, shortened version of 'I'm gonna').

There are a number of words that native English speakers get lazy with: "gonna" = "will", "going to" "I gotta" or "I've gotta" = "I must" or "I have got to" "skwout" = "Let's go out" "cheetchet?" = "Did you eat yet" And here's one you will hear only in Louisiana: "howzamommanem" = "How is your mama and them" (them = family)