A linear search goes through a sequence of items one item at a time from the start of the sequence until the item is found ...
sequence = 'abcdefghijklmnopqrstuvwxyz'
search = 'z'
for c in sequence:
if c == search:
print("found search item")
As you can see, if you were looking for 'a' it would be very efficient, but if you were looking for 'z' it would be the least efficient. If the size of your sequence is relatively small, a linear search is okay. However, imagine a sequence of a million or more items and your search item is near the end.