Argon2 is a key derivation function, or KDF. It is designed to be computationally-expensive and require a large amount of working memory in order to make brute force attacks more difficult. Internally, Argon2 uses a fast cryptographically-secure hash function called Blake2b.
A hash function takes an input of arbitrary length (the message) and maps it to an output of fixed length (the digest). When any single bit in the message changes, 50% of bits in the digest are expected to change. This effectively means that any change to the input results in a completely different output. Because the digest is a fixed size, the pigeonhole principle dictates that there will be some distinct messages which inevitably result in the same output digest. A cryptographically-secure hash function ensures that there is no mathematical way to predict what inputs will result in identical outputs. Any technique that makes finding such identical outputs easy is a collision attack.
A collision attack is any attack against a hash function f where a pair of inputs m ≠ m' is found and f(m) = f(m'). A 43-character base64-encoded output is 256 bits of data. Due to the birthday paradox, the collision resistance of an n-bit digest (a digest with a keyspace of 2n) is 2n / 2. This means the collision resistance of a 256-bit digest will be 2256 / 2 = 2128. A more mathematical explanation of hash function collisions and the birthday paradox is provided over at Crypto.SE.
In an information-theoretic sense, it would be more secure to use a larger output digest in that the output space would be larger. In practice, 128-bit collision resistance is more than enough.