To blether is to talk on and on about things that are not important. The word belongs especially to Scotland. A Scottish speaker might say someone is blethering when they are chatting, gossiping, or filling the air with pleasant nonsense. It can be a verb (to blether) and also a noun for the talk itself (a bit of blether). The feeling is usually light, not cruel: you can like a person and still say they like a good blether. Outside Scotland it sounds marked and folksy. In the rest of the English-speaking world people are more likely to say chatter, ramble, or blather — and blather is the close cousin, with a slightly ruder life in general English.