The colon means essentially “all elements in that dimension”:
julia> a = reshape(1:9, 3, 3)
3×3 reshape(::UnitRange{Int64}, 3, 3) with eltype Int64:
1 4 7
2 5 8
3 6 9
julia> a[1, :] # first row, all columns in that row
3-element Vector{Int64}:
1
4
7
julia> a[:, 1] # first column, all rows in that column
3-element Vector{Int64}:
1
2
3