What are decimation and interpolation in sampling rate conversion?

Decimation reduces sampling rate by factor M: keep every Mth sample, discard others. Requires anti-aliasing filter before decimation to prevent aliasing of high frequencies into lower bands. Interpolation increases sampling rate by factor L: insert L-1 zeros between samples, then lowpass filter to remove spectral images. Sample rate conversion by rational factor L/M combines interpolation by L then decimation by M. Efficient implementation uses polyphase filters. Applications: Multi-rate signal processing, audio resampling, and communication systems.