An algorithm is a precise set of steps for solving a problem. Searching is one of the most common problems there is: is this item in the list, and where?

The simplest algorithm is linear search: start at the beginning and check each item in turn until you find it, or run out of items.

def linear_search(items, target):
    for i in range(len(items)):
        if items[i] == target:
            return i
    return -1

names = ["Ada", "Grace", "Linus", "Alan"]
print(linear_search(names, "Linus"))
print(linear_search(names, "Bjarne"))

It returns the position, or -1 meaning "not found". It works on any list, sorted or not. Python's in and .index() do exactly this internally.