metric modulation sounds really technical, but it's a pretty simple concept: basically you are using some pulse from one section as the anchor for the time signature for another section. Like, say you are in 4/4 and you are using a lot of dotted eighth-notes at the end of a section, then your next section is in 3/4 but using the duration of that dotted eighth-note as the tempo for your new "quarter-note". So technically from the stand point of your previous meter, the new section would be in 9/8 rather than 3/4, but you would do this to avoid certain notational conventions... like say you are using dotted-eighth-to-16th-note rhythms or something. To do this in notation, you simply put a little note at the top where the "tempo" would go that has "a dotted-eighth = a quarter-note".
Obviously it can be used for much more complicated changes then what I'm listing here, but it's not more complicated than that. The whole idea is that it he ear doesn't really notice that you're abruptly changing tempos.