What is this liquid that comes out some time after urinating, is yellowish in color, not thick, and its trace is discovered on clothes after several prayers? And must the prayers performed before its discovery be repeated?

The fluid that usually comes out after urination is al-wady. It is a thick, white fluid that necessitates Wudu' (ablution) and contaminates whatever it touches. If a person prays with Wudu' but with clothes that were affected by al-wady without his knowledge, or due to forgetfulness, or ignorance, his prayer is valid according to the majority of scholars, and he does not need to repeat it. If he doubts its exit during or after the prayer, he should disregard the doubt and build upon the original state, which is purity. However, if he prayed after al-wady exited without performing Wudu', his prayer is invalid and must be repeated.