These grants are REALLY competitive!!
For example, the NSF and NIH fund around 30% of proposals that they receive.
You have to figure out where the gaps in knowledge are in your field, why they are important, & think about clever studies & experiments to adequately address those gaps
THIS IS A TON OF WORK!
If they are interested, they send it off to reviewers - other expert scientists, often in your field, who have volunteered to evaluate the validity of your study and its findings.
What it means is that published works that are peer reviewed are vetted works of science.
Here are some recent data on rates:
NSF: dellweb.bfa.nsf.gov/awdfr3/default…
NIH: nexus.od.nih.gov/all/2019/03/13…