ROL          ROL Rotate one bit left (memory or accumulator)          ROL

               +------------------------------+
               |                              |
               |   +-+-+-+-+-+-+-+-+    +-+   |
  Operation:   +-< |7|6|5|4|3|2|1|0| <- |C| <-+         N V - B D I Z C
                   +-+-+-+-+-+-+-+-+    +-+             / . . . . . / /

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Accumulator   |   ROL A               |   $2A   |    1    |    2     |
  |  ZeroPage      |   ROL $FF             |   $26   |    2    |    5     |
  |  ZeroPage,X    |   ROL $FF,X           |   $36   |    2    |    6     |
  |  Absolute      |   ROL $FFFF           |   $2E   |    3    |    6     |
  |  Absolute,X    |   ROL $FFFF,X         |   $3E   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+
  For penalty cycles on the 65816, check the desired addressing mode.

What it does: Rotates the bits in the Accumulator or in a byte in memory to the left, by one bit. A rotate left (as opposed to an ASL, Arithmetic Shift Left) moves bit 7 to the carry, moves the carry into bit 0, and every other bit moves one position to its left. (ASL operates quite similarly, except it always puts a 0 into bit 0.)

Major uses: To multiply a byte by 2. ROL can be used with ASL to multiply multiple-byte numbers since ROL pulls any carry into bit 0. If an ASL resulted in a carry, it would be thus taken into account in the next higher byte in a multiplebyte number.
Notice how the act of moving columns of binary numbers to the left has the effect of multiplying by 2:

0010    (the number 2 in binary)
0100    (the number 4)

0010    (the number 10 in decimal)
0100    (the number 100)