This paragraph leaves me confused. Can we try a different approach to explaining this? Here's a stab:

A "log stream" is a single, linear data stream that is spread across one or more sequential log files. For instance, an Apache or nginx server typically generates two log streams for each domain: an access log and an error log. Each stream might be written to a single log file that is periodically truncated (ick!) or rotated (better), with some number of historical versions being kept (e.g. access-example.com.log, access-example.com.log.0, access-example.com.log.1, etc.). Or, better yet, the server might periodically create new timestamped files so that the 'tip' of the log stream jumps from file to file (e.g. access-example.com-2014.01.28.log, access-example.com-2014.01.27.log, access-example.com-2014.01.26.log, etc.). The job of Heka's Logstreamer plugin is to understand the file naming and ordering conventions for single type of log stream (e.g. "all of the nginx server's domain access logs"), and to use that to watch the specified directories and load the right files in the right order. The plugin will also track its location in the stream so it can resume from where it left off after a restart, even in cases where the file may have rotated during the downtime.