In this poster for The Lobster we see Colin Farrell in an incomplete form, but in viewing the movie I don't remember seeing anything similar to what the poster is suggesting or related to it in any way.
What meaning is this poster trying to convey?
The film has two posters
The two beautifully minimalist posters of Yorgos Lanthimos’ dystopian movie The Lobster have been created by Greek designer Vasilis Marmatakis (cofounder, but no longer part, of MNP). The pair of posters feature once Colin Farrell, once Rachel Weisz, embracing a person-shaped void. - src
And from IMDb plot synopsis:
In a dystopian near future, single people, according to the laws of The City, are taken to The Hotel, where they are obliged to find a romantic partner in forty-five days or are transformed into beasts and sent off into The Woods.
So the poster suits quite well to the story as the void represent the missing partner quite well.
Note: I have not seen the film but the poster complemented the IMDb synopsis quite well for me.
In addition to Ankit Sharma's answer, I think there's a second meaning. During the movie, the short-sighted woman (Rachel Weisz) loses her sight. She's no longer able to see the person she loves. (And she loses it because she loves him.) It's implied at the end of the movie that David (Collin Ferrel) is going to blind himself, leaving him also unable to see his lover. Once they're blind, they will only be able to feel their lovers embrace, but not actually see them. I think the poster represents that fairly well.
The Lobster is actually absurdly humorous for a while. It takes a turn to the dark side and never comes back. In spite of the dark content, it does still manage to coax a smile at the absurdity of people, now and then. I feel like the underlying theme is loneliness and isolation. The poster captures that feeling.
The answers above are correct. I would like to add that the shape of the arms of the protagonist resemble claws of a lobster.