A lace frontal is used for someone who has a thinning or receding hairline, but has biological hair elsewhere. It replaces/covers the front hairline, allowing for a full hairline appearance. Typically a frontal is bonded at the hairline and clipped or sewn at the back. This is a great hair replacement option for someone who wants to "wear their own hair" but has a compromised front hairline.
You will notice that lace frontals are not substantially less expensive than an actual full lace wig. Therefore you may want to consider whether a frontal is the best option for you.