This tool analyses your text according to the Academic Word List or AWL (Coxhead, 2000). You can find a full description of the AWL at the Victoria University, New Zealand website here. It is the most widely used and recognized Academic word list.
The tool also analyses your text in terms of key Phrases identified by Martinez and Schmitt (Martinez and Schmitt, 2012), identified as K1, K2 and so on. If a phrase appears in the K1 list, that means it is in the first 1000 lexical items in the British National Corpus, so in theory is quite frequent and therefore probably easier for language users
The PHRASE List is inclusive of phrasal expressions in the British National Corpus (BNC) to a frequency that matches the frequency of the top 5000 words, also from the BNC. This frequency threshold has been identified as having functional significance for single words (e.g. the ability to read and write at good levels of general proficiency), and the items in the PHRASE List provide one missing lexical element that previous wordlists generally had not accounted for.
You can find a full discussion of the Phrases list, including the User guide from which this quotation was taken, on Norbert Schmitt’s website here.
Coxhead, Averil (2000) A New Academic Word List. TESOL Quarterly, 34(2): 213-238.
Martinez, R. and N. Schmitt, (2012). A Phrasal Expressions List. Applied Linguistics 33, 3: 299-320. Download it here.