A way of lumping together constant bitrate
or fixed bandwidth pipes together in a standard fashion.
telecos are in the business of providing fixed bandwidth voice pipes around. It is convenient for telcos to bunch together a standard number of those independent voice channels together. For example, a big-ish company may buy a primary rate ISDN E1 (T1 for the US) line from a telco. E1 has 32 independent 64 kilobit channels, giving the company in question some 30 useable telephone lines in one convenient wire. (cf. BISDN)
Of course, telcos have many customers like that, so it is convenient to bunch together many E1 lines. This can go on over and over, leading to the industry's development of SDH type technologies. Several exist, but the basic idea behind them all is the same.
SONET, or Synchronous Optical NETwork is the standard developed by US telcos in an attempt to standardise fibre based communications. The Europeans have a similar but not quite 100% compatible standard called SDH. As it is, many implementations are capable of understanding both.
The various SDHs are just link layers. They can carry various kinds of traffic, including voice, ATM and PoS.