Many jobs have the same shape: set something up, use it, then tidy up: open a file, read it, close it; take a lock, change shared data, release it; start a timer, run some code, stop the timer. The tidy-up step must happen even if the middle part crashes, or files stay open and locks are never released.

Done by hand, that needs try/finally:

f = open("notes.txt", "w")
try:
    f.write("hello")
finally:
    f.close()
print(f.closed)

with packages the pattern up. An object that knows how to set up and tidy up is a context manager, and with calls its setup on the way in and its tidy-up on the way out, whatever happens:

with open("notes.txt", "w") as f:
    f.write("hello")
print(f.closed)