What is Trande analysis Curve how to prepare it ?

Trend analysis means analysis of tendencies. Usually you collect data at different points in time and analyse curves.
Examples:
1) analysis of success probability trends - you calculate probabilities of meeting project targets during project execution and create curves. Negative (when success probability goes down) tendencies (trends) show that corrective action is necessary,
2) analysis of CPI (Cost Performance Index) and SPI (Schedule Performance Index) trends. If they go down then project performance became worse.
We always recommend to analyse trends. It helps to notice negative tendencies early and to apply necessary corrective actions until it became too late.