When we are talking ontically we sometimes use the expression 'understanding something' with the signification of 'being able to manage something', 'being a match for it', 'being competent to do something'.1 In understanding, as an existentiale, that which we have such competence over is not a "what", but Being as existing. The kind of Being which Dasein has, as potentiality-for-Being, lies existentially in understanding. Dasein is not something present-at-hand which possesses its competence for something by way of an extra; it is primarily Being-possible. Dasein is in every case what it can be, and in the way in which it is its possibility. The Being-possible which is essential for Dasein, pertains to the ways of its solicitude for Others and of its concern with the 'world', as we have characterized them; and in all these, and always, it pertains to Dasein's potentiality-for-Being towards itself, for the sake of itself. The Being-possible which Dasein is existentially in every case, is to be sharply distinguished both from empty logical possibility and from the contingency of something present-at-hand, so far as with the present-at-hand this or that can 'come to pass'.2 As a modal category of presence-at-hand, possibility signifies what is not yet actual and what is not at any time necessary. It characterizes the merely possible. Ontologically it is on a lower level than actuality and necessity. On the other hand, possibility as an existentiale is the most primordial and ultimate positive way in which Dasein is  characterized ontologically. As with existentiality in general, we can, in the first instance, only prepare for the problem of possibility. The phenomenal basis for seeing it at all is provided by the understanding as a disclosive potentiality-for-Being.
Possibility, as an existentiale, does not signify a free-floating potentiality for- Being in the sense of the 'liberty of indifference' (libertas indifferentiae) . In every case Dasein, as essentially having a state-of-mind, has already got itself into definite possibilities. As the potentiality-for-Being which is is, it has let such possibilities pass by; it is constantly waiving the possibilities of its Being, or else it seizes upon them and makes mistakes.3 But this means that Dasein is Being-possible which has been delivered over to itself—thrown possibility through and through. Dasein is the possibility of Being-free for its ownmost potentiality-for-Being. Its Being-possible is transparent to itself in different possible ways and degrees.
Understanding is the Being of such potentiality-for-Being, which is
1 '... in der Bedeutung von "einer Sache vorstehen können", "ihr gewachsen sein", "etwas können".' The expression 'vorstehen' ('to manage', 'to be in charge') is here connected with 'verstehen' ('to understand').
2 '... von der Kontingenz eines Vorhandenen, sofern mit diesem das und jenes "passieren" kann.'
3 '... ergreift sie und vergreift sich.'