What an activity is
An activity is a record of contact — a call, an email, a meeting — attached to the lead, deal or client it concerns.
Activities are recorded against the record they belong to, so opening a client shows the history of dealing with them rather than requiring somebody to remember it.
Why it is worth the thirty seconds
The value is not for you today; it is for whoever picks the relationship up next — including you in six months.
A customer who has to explain their situation again because the person who knew it was unavailable is a customer being told their history was not worth writing down.
Log it at the time
After the call, not at the end of the week. Reconstructed notes are both slower to write and less accurate, and the detail that mattered is the first thing lost.
What to write
What was discussed, what was agreed, and what happens next. The third one matters most: an activity that records a commitment without recording who owes what by when is a note, not a handover.
Using it
Before any call, read the last two or three activities on that record. It takes under a minute and is the difference between continuing a conversation and starting one over.