It is well known that utterances may be formed by one or more sentences and somebody can determine where one sentence ends and another begin. In the same way, in phonology, utterances can be divided into tone-units. Someone can determine where these tone-units begin and where they end.
There are cases where a clear pause makes determining the boundary between tone-units very easy but how can you determine where the boundary is if there is no clear pause.
One way to determine the boundary between two tone-units is to detect the change from the pitch level at the end of one tone unit to the pitch level that starts the following tone unit. All speakers tend to return to a particularly pitch level at the beginning of a tone-unit. Identifying that pitch level means identifying the boundary between tone-units.
Another way of identifying the tone-unit boundary is by analyzing the rhythm of speech because within the tone-unit speech has a regular rhythm. That rhythm is interrupted at each tone-unit boundary.
These are two ways of identifying the tone-unit boundaries but sometimes it is very difficult to make a clear decision regarding tone-unit boundaries when analyzing natural speech.