Amplitude refers to the size of a sine wave. It can be defined in various ways, but a standard definition is that it is the maximum value of a wave's departure from its average value. (The average value of a sine wave lies midway between its peaks and troughs.) The size of a sine wave is sometimes also expressed as a peak-to-peak amplitude, which is the vertical distance from peak to trough.
Root-mean-square (r.m.s.) amplitude is a way of specifying the size of a sine wave so that comparisons can more easily be made with steady, non-oscillating sources. The r.m.s. amplitude is the amplitude as defined above multiplied by approximately 0.71 (you do not need to memorise this figure). A steady source equal in value to the r.m.s. amplitude of an oscillating source supplies energy at the same rate as the oscillatory source, other things being equal. Root-mean-square amplitudes are often met in electrical contexts.