VirginiaTam: yes, you're right. A tunes session is an informal gathering, usually free and not advertised, of musicians playing sets of mainly traditional dance tunes from a common repertoire. People join in tunes they know and pick up the ones they don't by ear, or go and hunt for the dots or a recording later. All acoustic instruments are welcome; electric ones generally don't fit in so well, although there are honourable exceptions.

There are often tunes sessions at festivals. Otherwise they are mostly in pubs.

Usually people take turns to lead a set of tunes, sometimes by invitation and sometimes just by plunging in, depending on whether any individual is steering the session or not.

Beginners are generally welcome to experiment quietly in the back row, where they stand a better chance of being able to hear themselves. Getting a book of dots out in a full-speed session is a bit unproductive, because by the time you find the tune you want the others will probably have moved on to another one, and anyway you need to watch whoever's leading to get indications of speed and whatever signal is being given to herald a change of tune. It's best to try to pick out the bones of a tune by ear in the session and put in the work with the dots at home in private.

Finally, about 75% of English tunes are in G and most of the rest are in D. If someone's leading on a one-row melodeon they are most likely, although not certain, to be in C. Scottish fiddlers love A.

I'm sure other people will have more helpful thoughts to offer and I'm not claiming that what I've said applies to absolutely any session anywhere.

Above all, session playing is very good fun and an excellent social activity.

Good luck,

Valmai (Lewes)