Conv1D layer input and output
As described on the linked page, 128 is the dimensionality of each vector (i.e. the number of input channels), and 10 is the number of timesteps.
8 is the resulting number of timesteps after applying the filter of size 3 to the initial 10 timesteps.
In order to make it clearer, let's visualize the 1D convolution (source):

The missing parts in the picture are the batch size (4) and the output time length (8).