What is the difference between sequential and concurrent tasks?

'sequential' means 'one after another' - first do task A, then do task B, then do task C...

'concurrent' means 'at the same time' - do tasks A, B and C all at once.

In a relay race, a team runs sequentially - each team member runs, then passes the baton to the next so that s/he can tun - but all the teams run concurrently - all the teams have someone running at any given time in the race.