Relabeling in Prometheus is a versatile tool that enables the classification and filtering of targets and metrics by modifying their label sets, which is crucial for managing Prometheus metrics and applies to Grafana Agent users as well. Labels, which are key-value pairs, help in organizing and understanding what's being measured, while relabeling rules can be applied at different stages of a metric's lifecycle, from selecting scrape targets to storing data in Prometheus' database and sending it to remote storage. The process involves configuring relabeling steps in various parts of the Prometheus configuration file, such as scrape_configs, metric_relabel_configs, and write_relabel_configs, each serving different purposes. A relabel_config block comprises several fields, including source_labels, separator, target_label, regex, modulus, replacement, and action, which can perform actions like keep, drop, labelkeep, labeldrop, replace, hashmod, and labelmap to manipulate label sets. These actions can filter targets and metrics, modify label values, scale Prometheus horizontally, and map label pairs to new names, with common use cases including ignoring applications, splitting targets across servers, and managing high-cardinality metrics.