0 used to refer to a space used for storing things that is large enough for a person to enter and walk around in:
a walk-in wardrobe
a walk-in refrigerator
1 used to refer to a place that you can go to without an appointment (= an arrangement made in advance):
2 someone who goes to a place without an appointment (= an arrangement made in advance):
Although the restaurant will take walk-ins, reservations are recommended.
With the track sold out and no walk-ins allowed, a crowd of about 51,000 is expected.
3 large enough for a person to enter and walk around in:
a walk-in closet
4 allowing people to come without planning to do so in advance:
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.