R's DenDF: Satterthwaite's denominator degrees of freedom.
R's F value.
Whether the rows were tested jointly: always true here.
R's Mean Sq: F σ².
R's messages (none so far).
R's NumDF: the rank of the hypothesis.
R's Pr(>F).
R's Sum Sq: F σ² NumDF.
R's warnings (about the Hessian's eigenvalues; Contrast is rank deficient and test may be affected).
Result of lmerContest with
joint: true(the default): one row of an F table, or none when the rows of L have rank 0.