Start of date/time range used to analyze data. End of date/time range used to analyze data.
Within Autoresponder Max, all data that is used requires a set of date fields. For example, an order has an order date and a shipped date, besides others. These date fields are used and must fall within the date range set by Time Frame Start and End within each email campaign. This data range is how long ago the event occurred (e.g. how many days ago an order was placed). Time Frame Start and End just represents a date range, the Time Frame Field is the actual date field (e.g. order date, shipped date) that is used for comparison.
This is probably best explained with an example. If an email campaign is desired to send to new orders 3 days after they placed, the settings Time Frame Field would be set to Created, Time Frame Start set to 3 days, and Time Frame End set to 4 days. Orders that were placed between 3 and 4 days ago will be retrieved and processed to possibly send an email to. Orders that either too new or too old are simply just ignored.
If Time Frame Start and End are both set to blank, the range of time will be essentially be immediate, so anything new will be retrieved and processed.