The Summer Triangle is an asterism (またはconstellation)with lines drawn between the stars Altair, Deneb, and Vega forming a triangle
「夏の大三角形」= the Summer Triangle / the Summer Triangle asterism
もっと詳しく説明したいなら、こちらの文章を使えます:
The Summer Triangle is an asterism (またはconstellation)with lines drawn between the stars Altair, Deneb, and Vega forming a triangle