Complex sentence: A man-at-arms was a soldier from the High Medieval to Renaissance periods who was typically well-versed in the use of arms and served as a fully armoured heavy cavalryman.
Split into simpler sentences: A man-at-arms was a type of soldier of the High Medieval to Renaissance periods. He was almost always a professional warrior in the sense of being well-trained in the use of arms, who served as a fully armoured heavy cavalryman.