(Mishnah): One may not use bundles of straw, wood or cane as Schach.

If he untied them, they are valid.

They may all be used as Sukah walls.

(Gemara - R. Chiya quoting R. Yochanan): Bundles are invalid because it looks like he is storing the bundles on top of the Sukah. (Chazal were concerned that in the summertime a person might come in from the field carrying bundles of fresh moist sticks and will put them on top of his Sukah to dry. Later, when it comes to Sukos, he might just decide to use those bundles as his Schach (without moving them). This would completely disqualify the Sukah as it is "Taaseh Velo Min Ha'asui'' - that the Schach was not originally put there for shade.)

R. Yaakov bar Avaya/Rav Sheshes quoting R. Chiya Rabah: It is not called a bundle if it contains less than 25 sticks.