A "lightweight sweater" refers to a thin and not very heavy type of sweater made with light materials. It is not as warm or thick as a regular sweater but still provides some warmth. Full definition
Add a cotton tee or shirt with lightweight sweater on top, preferably wool or cashmere. (the-redhand.com)
I love lightweight sweaters for those days when it's warm during the day but cooler in the evenings. (ladyinviolet.com)
Either way, it's become of my favorites lightweight sweaters in my wardrobe, and I frequently wear it on the weekend with boyfriend jeans. (twotakeonstyle.com)