solve error

w

[,1] [,2] [,3] [,4]
1:1  0.0  0.5  0.5  0.0
2:1  0.5  0.0  0.0  0.5
1:2  0.5  0.0  0.0  0.5
2:2  0.0  0.5  0.5  0.0

solve(w)

Error in solve.default(w) : Lapack routine dgesv:
system is exactly singular

what does the error mean?