How to identify what class they correspond with?
We can get this from the class names property of the output layer.
What these probabilities mean?
They are the probability that the given image is in the corresponding class.
Here's an example
net = squeezenet;
img = imread('peppers.png');
img = imresize(img,net.Layers(1).InputSize(1:2));
We can get the names of the classes from the output layer, which happens to bne the last layer for this network:
c = net.Layers(end).Classes;
Then get the activations from the softmax layer
p = activations(net,img,'prob','OutputAs','rows');
Note that these sum to 1
ans = single
We can then find the maximum probability and which class that corresponds to
pm = single
i = 946
ans = categorical
This is the same information that comes from the classify command:
[cc, pp] = classify(net,img);
cc = categorical
ans = logical