What does volume mean? When we talk about OHLCV bars for say LTC-BTC, what does volume mean ?

Is it the quantity of LTC transacted during the bar or the quantity of BTC transacted during the bar?
An LTC-BTC Trading pair is the buying of LTC for bitcoins, so the volume of this market will indicate how much LTC has been bought for bitcoins.

The total amount of this trading volume can be shown in different value, if you want you can simply divide this amount by its adjusted price of LTC to know how much LTC has been transacted on this pair.

Usually the volume will be shown in dollar, so if we have a BTC at 10000 dollar, an LTC at 100 and the volume LTC-BTC show 1000 dollar we do:
1000/100 = 10

10 LTC has been exchanged for bitcoins.