In the first chapter of the Torah G-d Himself blesses various things: people, animals, days.
What does it mean for G-d to bless anything after creating it?
According to Nefesh haChaim shaar 2 (mostly around chapter 2, but it's a major theme of the whole section), a berakhah is a way to "draw down" the spiritual light inherent in the physical object.
For example, R Chaim Volozhiner writes that when the gemara says that someone who eats without making a berakhah is akin to stealing, it doesn't mean that they stole that apple (example mine) from G-d by using it without the Maker's permission. Hashem gave us permission, "the heavens -- the heavens are for G-d, but the land He gave to Adam's descendants." (Tehillim 115:16)
Rather, he says the person is stealing from the world the spirituality that could have been brought out by acknowledging the Divine Source of the apple.
The Zohar (and Rashi, and the Ritva, and...) say that "berakhah" is a term that means increase. By acknowledging the G-dliness inherent in the created thing, we increase the amount that G-dliness is manifest in the world.
And if you see that manifest G-dliness through awareness in terms of a commitment to use that apple's energy to actively bring G-d's Will into the world, you get R SR Hirsch's explanation described in Saba Hillel's answer. But unsurprisingly, R Chaim Volozhiner's explanation is in more mystical terms.
Now to answer your question... In Bereishis 1, Hashem's blessing things would therefore mean that He left access available in this physical universe to the spirituality inherent in His causing that apple to exist.
Rav Hirsch explains in Bereishis 1:22 that the term blessing with respect to all creatures except Man refers to the automatic fulfillment and the refinement of the creation as part of the command. That is, this was part of the creation and is specified to explain what is being done and what details needed to be included.
Hashem blessed the organic living creatures, i.e. He gave them the power of reproduction and multiplying, and with this power He gave them at the same time the direction and urge for it. For in the unfree living organic beings, power and fulfillment are not separated. The power itself automatically drives them to the fulfillment of that for which Hashem has given them that power.