Resampling is a method doing one of these: bootstrapping(random resampling), jackknifing(using subset of data), permutation test(or exact tests, randomization tests or re-randomization tests), cross validation.

By taking many smaller samples from the given samples, one can estimate confidence interval:
Statistics and Data Analysis: Confidence Intervals Based on Resampling

Here’s R code for it:
Quick-R: Bootstrapping

Resampling can be used for other purposes like Permutation test, Cross Validation, Simulation, etc. Explanation on these can be found at:
The Statistical Bootstrap and Other Resampling Methods
Permutation test