In percussion terms, the part of the rudiment where not much is going on. An inner beat isn't flammed or accented or diddled, it's just the unornamental but very important part of the rudiment. Example:

 >    *   *    |   >     *    *   ||
(Rl)  l   r    |  (Lr)   r    l   ||
               |                  ||
|____ 3 ____|      |____ 3 ____|

In the flam accents notated above, the notes with * over them are inner beats. Likewise, in these flam drags:

 >           *    |   >           *    ||
(Rl)  (ll)   r    |  (Lr)  (rr)   l    ||
                  |                    ||
|______ 3 _____|      |______ 3 ______|

Even the most complicated rudiments have inner beats. Note the Inverted Cheese Chewbaccadiddles Off the Left Hand below:

         >   *          |          >   *           ||
({rr}l) (L) (r) (l) (l) | ({ll}r) (R) (l) (r) (r)  ||
                        |                          ||

To any aspiring percussionist, an experienced one would offer this advice: learn your rudiments until you know them like your own face, and then, play just the inner beats. Once you can do that, you are a percussionist.