The difference between "iambic meter" and "iambic pentameter" is the difference between general and specific. First, "iambic" refers to verse written in iambs. Iambs are a rhythm that is composed of one unstressed beat followed by one stressed beat. In an allusion to musicians, you might say it is poetry written in 2/2. You might also say iambs are a duple rhythm. Iambs are scanned, in poetic scansion, like this [ - ' ], that is, unstressed stressed.
Meter refers the combination of rhythm plus line length. Poetic lines are measured in "feet" in a way similar to how lines of music are measured in, well, measures. Poetic lines can have various lengths. In other words, poems can have lines of few feet or many feet.
Now, when we say a poem is in iambic meter, we are identifying only the rhythm. We have alluded to a meter but have not named, or counted, the feet in the poetic lines. In other words, there may be two feet of iambs or more, like eight feet of iambs.
In contrast, when we say a poem is in iambic pentameter, we are identifying both the rhythm and the line length. Pentameter designates a very specific number of feet. Pent is a Latin word meaning five. When pent is joined to the word meter, as in pentameter, we immediately understand (after we have memorized this, anyway) that the poetic line has five feet. Therefore, iambic pentameter tells us that the iamb rhythm carries on for five feet. This would scan like this: / - ' / - ' / - ' / - ' / - ' /: five groups of iambs in sections called feet.
A line in iambic pentameter might sound something like this: "A horse' / is built' / to run' / for fun' / of course'." A line in iambic meter might look like this, "Oh boy', / for joy'!" or it might look like this: "To build' / upon' / dear dreams' / the boy' / may ride' / the horse' / today' / to win' / the crown'." The rhythm is specified but the line length is left unspecified when all that is said is "iambic meter."
In poetry, a foot is the basic metrical unit of a verse (i.e. line of poetry). An iambic meter refers to a foot consisting of an unstressed syllable followed by a stressed syllable (i.e. a-bove).
There are different types of iambic meter, one of which is called iambic pentameter. Penta is the Greek number prefix meaning five, thus iambic pentameter is a poetic line consisting of five iambic meters (aka iambic feet).