As far as the Camino Frances in concerned the Meseta is the section between Burgos and Astorga, approximately.
It is the name for the flat plains on the plateau of central Spain.
The heat will be no worse there than many other places and certainly less than on the same meseta sections on the Via de la Plata further South.
You do need to take care:
Yes it can be hot and on the meseta there is very little shade so take a hat and sunscreen.
Carry plenty of water, by then you will know how much is enough for you as some people seem to drink more than others I will not suggest how much.
Some of the sections on the meseta are longish without the villages that you can stop in to refill.
I am about to embark on this section of the Camino de Frances and the stories about heat and shade differ from every person who tells them
More correctly, what you're referring to is the meseta central, the central plateau of Spain, or high plains of Castile.
Having said that, the temperature on the first day was 33 degrees uphill. No one on this trip with any experience has the suggested that what's coming next could be worse.
Where can one buy these clip on umbrellas?I find an umbrella, rigged hand free, invaluable; it's a way to carry my shade with me. Mine is silver coated so particularly good in the sun. No good if it is windy, but if that happens the breeze will cool you. The umbrella is also very handy when it rains and you don't want to get into waterproof clothes (otherwise known as saunas!)