First of all, nice job on your productions, I like what you're doing so far. Regarding your question:
A "B section" is a descriptor for a part of a song which changes from the (you guessed it) "A section." When discussing musical form (a.k.a. song structure), we can think of different sections of music (usually with different harmonic content and different lyrics) as either designated with letters, like A-B-C-D (as in AABA form), or by terms such as verse, chorus, bridge, intro, outro, modulation (usually a chorus that changes key near the end of the song, to keep listener interest), etc. Every song has some kind of structure, or different parts that happen in different places; some repeating so that we hear something familiar to latch onto (the chorus; also known as the refrain or the "hook"), others that change (at least melodically and lyrically), so that the songwriter can get his or her story across. The letter designations give you a "road-map" of the song-- the layout, the way the various parts fit together to create its structure or overall form. So a B section is just a different section of the music.
Every good song should have a balance between repetition and change (both musically and lyrically); this is regardless of various small arrangement touches that add interest and act as "ear candy," but don't fundamentally change anything in the music. If we take the example of some of your instrumental productions so far ("Swagin' Out," "Fun Tonight," and "Time Has Come," for instance), you are doing a good job setting-up a groove, adding nice little elements so your "beats" have musical changes in them throughout, but if we boil them down to their essences, they are really just four-chord, four-bar loops that repeat again and again. If someone mentioned to you that you need a "B section," they mean a whole new part that has different, but related, chords and other musical elements. Some Hip-Hop production is still based on fairly simple formulas, but a lot of producers are catching on to writing more complex, and more traditional, song-like structures. That means that your chorus has to be clear and distinctly different from the rest of the piece, and it has to be really obvious so that folks know it's the chorus-- not just the same music again with an added sound effect, clap track, or short piano melody fragment.
Listen to lots of, and a wide variety, of songs (not just rap instrumentals). You will hear a lot of the same forms and variations on them used over and over again. The reason is that they work, and people need to hear something that repeats (to feel familiar with the piece) and things that don't repeat (to keep from going crazy while they listen). If you analyze various song styles and structures, it will take your musical creations to "another level," as they say.
Don't get A sections, B sections (C, and D, etc.) confused with what you often see on written sheet music. The various letters that designate sections in a lot of written charts do not necessarily coincide with verses, choruses, etc. (although they might)-- they are called "rehearsal letters," and they are just easy spots for a conductor to bring a group back to when he or she says "Let's take it from letter C, the second time through;" they are just a reference for reading and practice.
GJ
