This apparently simple phrase is actually quite interesting. First, it’s a good chance to practise the tricky /ð/ sound. Remember the front of your tongue must touch the top teeth, but don’t stick it out. The /ɪ/ sound must be nice and short. Practise going from /s/ to /n/ without making a vowel in between. Also, the /n/ sound must be further forward than usual, again, with the tongue just touching the top teeth, ready for the following /ð/ sound.
The phrase is a vague answer when there’s no need for detail, for example:
“What have you been doing?”
“Oh, you know, this and that.”